1. Field of the Invention
The present invention relates to a system for moving magnetic tape past a read/write head at a constant velocity by separately servo controlling the source reel motor and the take-up reel motor.
2. Description of the Prior Art
Present day personal computers often include hard disk drives capable of storing very large quantities of data, typically 20, 40, or 80 megabytes. If the hard disk drive should "crash" or malfunction, this data may be irretrievably lost. Reconstruction of the data from its original input at best is tedious, and at worst may be impossible if the original input records are no longer available.
Accordingly, it is desirable periodically to archive the contents of the hard disk drive. Magnetic tape is a particularly desirable media for such archival storage. Recently certain magnetic tape cartridges have been introduced which are well suited for this purpose. The cartridge itself contains a single tape-containing reel. It is utilized in a drive system in which the leader end of the tape first is pulled from the cassette and attached to a take-up reel that is external to the cassette. An appropriate drive system controls reel rotation so as to achieve the desired tape velocity past a read/write head. The present invention, although not so limited, is well suited for controlling the reel drive motors in such a cassette tape system.
In view of the very large amounts of data which typically must be transferred from a hard disk to a cassette tape for archival purposes, it is desirable to maximize the read/write data transfer rate to the magnetic tape. To accomplish this with maximum accuracy requires that the tape be transported past the head at a tightly controlled, constant velocity. An objective of the present invention is to provide such a constant velocity tape drive system.
Direct measurement of the tape velocity past the head is impractical. For example, such direct velocity measurement could be achieved by having the tape pass in contact with an idler wheel, the rotational rate of which would directly indicate tape velocity. However, this would require the use of mechanical elements, such as the idler itself, with which the tape must be held in good contact to assure accurate velocity measurement. Such mechanical arrangement has the deleterious effect of introducing unwarranted drag tension forces on the tape, thereby potentially complicating rather than simplifying the overall control problem.
Source or take-up reel rotational velocity can be measured with reasonable accuracy. However, to calculate the linear tape velocity past the head from the measured reel angular velocity requires knowledge of the current tape radius on the reel. This radius changes continuously as tape is transferred between the source and take-up reels. Thus reel radius information also must be obtained continuously if the reel angular velocity is to be used to measure and control tape speed past the head. Another object of the present invention is to provide a system in which such reel angular velocity and tape radius are continuously and accurately measured and utilized in a constant velocity tape drive system.
One approach to such angular velocity tape radius measurement is presented in the U.S. Pat. No. 4,125,881 to Eige et al. In that system the source reel is provided with an encoder which emits N pulses during each revolution of the source reel. The number of such pulses emitted during a single revolution of the other, take-up reel is counted. From this count value, the present tape radius can be calculated using a certain "radius constant". That constant is related to the radius of the reel hub and the maximum radius of the tape when all of the tape is wound on only one reel.
In the system of the U.S. Pat. No. 4,125,881 such radius constant and single encoder count value are used to obtain current tape radius values for both the source and take-up reels. The time between occurrence of the single encoder "fine tach pulses" is compared with a known time period that should occur if the tape is moving at the correct velocity. Any difference represents a velocity error. This error value is used as a common input to the servo drivers for both the source and take-up reel motors.
A system in which a single fine encoder is used, situated on one or the other of the source and take-up reels, has certain inherent shortcomings. When most of the tape is on the reel having the encoder, only a very few pulses are emitted during one revolution of the other reel. As a result, radius calculation accuracy is substantially reduced as compared with the accuracy which is achieved when most of the tape has been transferred to the other reel. Under that latter condition, many pulses will be emitted from the encoder during one revolution of the other reel, with concomitant higher radius calculation accuracy.
Thus in such a prior art system, radius measurement accuracy varies greately depending on how much tape has already been transferred from one reel to another. As a result, the accuracy with which linear tape velocity control can be achieved will vary depending on how much tape has been transferred.
This problem is compounded when the angular velocity measurement is taken into account. Since the time between encoder pulses is used as a measurement of reel angular velocity, this measurement likewise will be affected by the amount of tape presently on the reel. With small amounts of tape on the reel, consecutive encoder pulses will occur very rapidly, thereby reducing the accuracy with which the period can be measured.
It is a further object of the present invention to eliminate these shortcomings of the prior art by providing a constant velocity tape drive system in which separate fine encoders are provided on both the source and take-up reels, and in which information is utilized from one or the other of these, depending on which is presently providing the more accurate information.
Another factor to be taken into account in a tape drive system of the type described is the tension on the portion of the tape extending between the reels. Advantageously this tension should be held constant. If the tension varies, undesirable fluctation of the tape may occur. This may cause concomitant tape speed fluctations past the head, or could even cause irregularities in tape contact with the head.
In the above mentioned U.S. Pat. No. 4,125,881 a separate mechanical tension transducer is used to sense mechanically the tape tension between the two reels. A tension sensing arm provides a tension error signal that is supplied to the drivers of both motors.
Another object of the present invention is to provide a constant velocity tape drive system which includes an open loop tape tension control without using a mechnical tension sensor.